Gasoline refilling stations are typically equipped with vapor recovery systems to reduce emissions of hydrocarbon vapors during refilling of motor vehicles. Such systems vary in their details, but usually comprise a vapor line either concentric around a fuel line, or a second tube extending to near a fuel outlet nozzle. Vapors are drawn through the vapor line at a rate that can exceed the volumetric rate at which gasoline is pumped through the fuel line. A portion of the vapors removed from the vehicle's fuel tank are routed back to the fuel storage tank at the filling station to minimize the amount of vapor eventually vented to the atmosphere, and any vapors vented to the atmosphere are typically passed through an activated carbon filter.
It is also common for a fuel nozzle to be equipped with a seal that mates with a vehicle's fuel inlet to ensure that gasoline vapors do not escape from the fuel tank, and to provide a closer balance between the amount of vapor removed from the vehicle's fuel tank and the amount of vapor needed to maintain pressure in the fuel storage tank at the filling station.
Common problems with vapor recovery systems include overfilling of fuel tanks, resulting in liquids entering the vapor recovery system, and the difficulty of knowing when a seal between the fuel dispenser and the fuel tank inlet is achieved.
Numerous apparatuses have been proposed for preventing over-filling of fuel tanks during refuelling. The most common used method is an automatic cut-off within a dispenser nozzle. Typically this automatic cut-off uses a vapor path from the nozzle outlet back to a venturi around the fuel flow path within the nozzle. A sufficiently high pressure must be maintained at a point within this path to indicate that vapor is being drawn into the vapor path rather than liquids. When liquids enter the vapor path, the pressure drop in the path increases, and the pressure at the sensor point will decrease. When this pressure decreases below a threshold pressure, the fuel flow is cut-off, usually by a mechanical trip. When a vapor recovery nozzle that seals the fuel inlet is incorporated with a vacuum assist vapor recovery fuel dispenser, this automatic fuel cut-off will not function properly because pressure at the pressure sensor is subject to variations due to variations in the vapor recovery system. Such variations result in the shut-off not having sufficient consistency.
An electo-mechanical fuel cut-off switch is disclosed in U.S. Pat. No. 5,131,441. This switch incudes an electromagnetic clutch that enables the trigger of a nozzle to close. When fluids are detected by a fluid actuated switch located in the nozzle spout, electrical energy to the electromagnetic clutch is interrupted, and the fuel valve is closed. This mechanism is said to be quick-acting, and therefore minimizes splash-back losses to the environment. An optical liquid sensor is suggested as the fluid actuated switch. The fluid actuated switch is located within a baffled channel in the nozzle in the fuel dispenser of '441. The location of the switch within the nozzle relies on the fluid level raising within the baffled channel of the nozzle. Because vapor is trapped within the closed volume of the nozzle, liquid will not necessarily back-up in the nozzle, but could raise outside the nozzle and be drawn into the vapor recovery system. Liquids could therefore be exiting the fuel tank into the vapor recovery system before the fuel flow is cut off by the mechanism of patent '441.
Another fuel dispenser having a vapor recovery system is suggested in U.S. Pat. No. 5,121,777. This dispenser includes a flexible boot surrounding the fuel nozzle that encloses a vapor recover conduit. The flexible boot includes an electro-mechanical switch that must be closed as one of three requirements for the fuel flow to be enabled. The electro-mechanical switch is closed when the boot is compressed against a vehicle's fuel inlet nozzle. The electro-mechanical switch requires that an electrical contact be made within the fuel dispenser right at the mouth of the fuel tank. It would be preferable to have a system that does not require an electro-mechanical switch because a such a switch can be unreliable, and because of the possibility that it could be a source of ignition.
It is therefore an object of the present invention to provide a method and apparatus for cutting off fuel flow in a fuel dispensing nozzle wherein a significant amount of fuel is not drawn into a vapor recovery system before the fuel flow is discontinued. In another aspect of the present invention, it is an object to provide a fuel dispenser wherein a determination that a sealing contact is made between the fuel dispenser and a fuel inlet of a vehicle without requiring a mechanical switch.